Pigmentation of Amphibians

In general, amphibians have a wide array of colors when it comes to skin pigmentation. There are approximately 6,400 species of amphibians (Crump 2009), and their unique colors and color patterns are characteristics that may be used to distinguish species from one another. The pigmentation of amphibians are determined by specialized cells called chromatophores. Chromatophores are cells that contain or produce pigments or reflect light in order to display a certain colors. Amphibians benefit from the colors that they display in a number of ways, ranging from maintaining body temperature to avoiding predators. For example, some amphibians may use their color to blend in with their environment to escape from predators, or brightly colored amphibians may avoid their predators by informing others that they are poisonous. Not only is there a number of different colored species, some species of amphibians may change their color even after development. In order to understand the coloration of amphibians, studies show the mechanisms behind how amphibians obtain their color during development and how some change colors depending on which environment they are in.

Common Tree Frog Poison Dart Frog

How do they get their color?

Chromatophores consist of several subclasses of pigment-containing cells that produce specific colors in amphibians. The three types of chromatophores that are important in the coloration of amphibians are: melanophores (contain pigments that appear black or brown from melanin), xanthophores (contain yellow colored pigments), and iridophores (contain reflective or iridescent pigments). The three types of chromatophores are localized such that the xanthophores are located uppermost, iridophores situated under that, and then the melanophores are located at the very bottom (Yasutomi and Yamada 1998). This special localization was termed as the Dermal Chromatophore Unit (DCU) (Bagnara et al., 1968). The DCU was found to form during metamorphosis. Although the mechanism for the organization of the DCU is unknown, they suggested that different migration rates of the three types of chromatophores may be a potential factor that determines their position (Yasutomi and Yamada 1998). The DCU is located under the basal lamina, which is a layer of extracellular matrix that the epithelium is situated on. Figure 1 below shows a schematic representation of the DCU.

It was suggested that chromatophores are formed from the neural crest (Twitty 1953). Twitty also noted that it seemed as if pro-pigment cells, or chromatoblasts, release chemical substances that allow cells to scatter until they are beyond the mutual influence range and to migrate to their resting places in the skin. Also, these cells have an excitatory effect on their neighboring cells. During development, the concentration of different pigments in the chromatophores gives the amphibians their color.

Axolotls, which are neotenic mole salamander that fails to undergo metamorphosis and remains aquatic and gilled, are commonly used for scientific research. Results from an experiment (DuShane 1935) showed that an albino axolotl, in contrast to a black axolotl, is not deficient in melanophores because they lack melanoblasts, which are precursor cells to melanocytes that produce melanin for their black color. However, DuShane suggested that the epidermis and mesoderm failed to allow differentiation of pro-pigment cells. In addition, DuShane observed that, when a piece of ectoderm from a black axolotl was transplanted onto an albino axolotl, a heavy patch of black pigment was formed. From the results obtained by DuShane, Twitty (1953) advocated that production of pigments within the chromatoblasts is dependent on the enzymes or chromogens provided by the epidermis and mesoderm. The differentiation of pro-pigment cells give rise to chromatophores which defines the color of amphibians. In a follow-up experiment (DuShane 1939), the increase in pigmentation for an albino axolotl was striking when both the ectoderm and mesoderm were replaced.

Although many observations were reported through these studies, many of the underlying mechanisms of DCU formation and pigment production are not fully understood. Further studies should focus more on the underlying mechanism and key factors, such as proteins. In addition, although DuShane suggested that the enzyme (dopa oxidase) possibly allowed pigmentation in the albino axolotl to occur, the actual enzyme has not yet been confirmed by other studies.

How do they change their color?

Many amphibians can change their dorsal color depending on the environment they are placed in. The color and reflectivity of the environment is important to which color the amphibian may change to. From measuring the color change of a treefrog, it was reported that treefrogs became lighter on brighter backgrounds and at higher temperature (King et al. 1994). Figure 3 and Figure 4 shows the change in treefrog brightness in response to change in background brightness and temperature. In order to change color, Bagnara and Hadley (1973) noted that there must be a dispersion and aggregation of pigment granules within the chromatophore.

King et al. 1994

The special localization of the chromatophores, or DCU, is also important in color change. From the results of another experiment (Yasutomi and Yamada 1998), it was reported that there must be a migration of the chromatophores underneath the basal lamina during metamorphosis from tadpole to frog. Within the chromatophores, the different types of pigments must translocate to obtain a change in color.

Example of Drastic Color Change. This shows the frog Oreophryne ezra changing colors from a dark, polka-dotted juvenile to a peachy colored adult with bright blue eyes. These frogs show a striking difference in color change and lose the benefits of having the “dangerous-poisonous characteristics” as they become adults. Color Change for Oreophryne ezra.